A process to back up data stored in a storage system to another storage system is known. In the conventional art, when the data to be backed up are updated while a backup operation is being carried out, the updated data are written to a storage region separate from a main storage region that stores the data to be backed up, and after the completion of the backup operation, the updated data are overwritten into the main storage region. However, according to the conventional art, during the time when the updated data that are overwritten into the main storage region, new data cannot be written into the main storage region, and this can lead to a processing delay.